Copy-by-value in sdbus++ generated objects

[Patrick Venture]

On Sun, Nov 4, 2018 at 6:36 PM Lei YU <mine260309 at gmail.com> wrote:
>
> On Sat, Nov 3, 2018 at 12:33 AM Patrick Venture <venture at google.com> wrote:
> >
> > Most repos inherit from an auto-generated object.  Those objects pass
> > all parameters by value.  This is fine for many cases where they would
> > need to create a copy of it, but in the case where:
> >
> > for instance:
> > https://github.com/openbmc/sdbusplus/blob/master/tools/sdbusplus/templates/property.mako.prototype.hpp.in#L69
> >
> > The values match it's wasteful to copy.  It's also possible the cpp I
> > learned way back is different in how this behavior works, but I wanted
> > to check.  Because of the prevalence of this code, if there are lots
> > of updates that don't actually change the value, it could optimize
> > quite a bit at run-time.
>
> Agreed, this piece of code does not change the value at all, so it is better
> to use const reference.
>
> >
> > It would require a lot of simultaneous fixes (or two different objects
> > generated, and transitioning everything).
> >
> > Thoughts?
>
> Maybe we could try to change this to const reference, do a build and see if it
> cause any build error.
> I guess this is OK.

So, I can guess without building that this change will generate a lot
of errors as all the overriding objects will be set to "override" but
not be overriding anything.  So, compilations will fail pretty much
across the board.  To be fair, it's actually maybe 20 repos that would
need fixing, so not that big of an order, but yeah, I don't think this
change can go in without a bunch of other simultaneous changes (unless
we add something like a suffix, '_next" or "_new_" or "_2" and then
slowly move things to calling that and then when everything is calling
that, make the original the same, and then move everyone onto the
original one.